Radioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide, transforming to an atom of a different type, named the daughter nuclide. A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons, its number of neutrons, and its excited state. Isotopes are different types of atoms of the same chemical element, each having the same number of protons and a different number of neutrons.